The present disclosure relates to a wireless communication technology field, and more particularly, relates to a method and apparatus for obtaining channel direction information.
To meet the demand for wireless data traffic having increased since deployment of 4G (generation) communication systems, efforts have been made to develop an improved 5G or pre-5G communication system. Therefore, the 5G or pre-5G communication system is also called a ‘Beyond 4G Network’ or a ‘Post LTE System’.
The 5G communication system is considered to be implemented in higher frequency (mmWave) bands, e.g., 60 GHz bands, so as to accomplish higher data rates. To decrease propagation loss of the radio waves and increase the transmission distance, the beamforming, massive multiple-input multiple-output (MIMO), Full Dimensional MIMO (FD-MIMO), array antenna, an analog beam forming, large scale antenna techniques are discussed in 5G communication systems.
In addition, in 5G communication systems, development for system network improvement is under way based on advanced small cells, cloud Radio Access Networks (RANs), ultra-dense networks, device-to-device (D2D) communication, wireless backhaul, moving network, cooperative communication, Coordinated Multi-Points (CoMP), reception-end interference cancellation and the like.
In the 5G system, Hybrid FSK and QAM Modulation (FQAM) and sliding window superposition coding (SWSC) as an advanced coding modulation (ACM), and filter bank multi carrier (FBMC), non-orthogonal multiple access (NOMA), and sparse code multiple access (SCMA) as an advanced access technology have been developed.
Multiple-Input-Multiple-Output (MIMO) technology can improve spectral efficiency of a wireless communication system through channel's spatial resources. In order to obtain corresponding spectral efficiency, a transmitter has to obtain Channel Direction Information (CDI) so as to perform pre-coding calculation and to process other MIMO signals. Channel State Information (CSI) includes the CDI and Channel Quality Information (CQI). For a MIMO system, it is a prerequisite to perform closed-loop MIMO transmission, and impacts the system performance that the transmitter obtains a precise CDI.
In a Long Term Evolution (LTE) system corresponding to an Evolved Universal Terrestrial Radio Access (E-UTRA) protocol defined by a 3rd Generation Partnership Project (3GPP), according to different duplex modes, there are different CDI obtaining ways. LTE duplex modes include Time Division Duplexing (TDD) and Frequency Division Duplexing (FDD).
In a TDD system, a same spectral resource is multiplexed for an uplink channel and a downlink channel, wherein there is Channel Reciprocity for the uplink channel and the downlink channel. Thus, when channel estimation is performed for the uplink channel, a TDD base station can obtain equivalent CDI of the downlink channel. In order to assist the channel estimation, a terminal transmits an omnidirectional Sounding Reference Signal (SRS) generated through a designated sequence that is convenient for the channel estimation and the pilot signal multiplexing, e.g., a Zadoff-Chu (ZC) sequence, a Pseudo-Noise sequence, wherein a pilot signal is known by the terminal and the base station. In a LTE TDD system, a shortcoming of obtaining the CDI through a way of SRS transmission and the channel estimation is a problem of pilot contamination. In the LTE system, SRS sequences assigned for different terminals in a same cell are orthogonal. According to the SRS sequences of the different terminals, a base station can perform non-interference channel estimation to obtain the CDI of the uplink channel. However, in the LTE system, SRS sequences assigned for different terminals in different cells are non-orthogonal. When estimating the CDI of an uplink channel of a terminal in a cell, a base station can be disturbed by uplink SRS signals from terminals of another cell. That is, the CDI of the channel of the cell estimated by the base station includes CDI from the terminals of the other cells to the base station, which is referred to as a problem of the pilot contamination. The pilot contamination will cause serious consequences for uplink data transmission and downlink data transmission.
1) When the base station transmits data through a directional pre-coding on the downlink channel for a desired terminal, it is equivalent that the base station transmits data of the direction to terminals of a neighbor cell on an overlapped channel, and the data of the direction is a serious interference among cells.
2) When the base station processes received data transmitted directionally by the desired terminal on the uplink channel, the base station also perform enhancement processing for data transmitted by terminals of a neighbor cell on an overlapped channel, thus, interference of the overlapped channel is amplified.
According to reasons above, the pilot contamination impacts system throughput. When the number of antennas is increased, a bottleneck of improving system performance may occur.
A Large-scale MIMO (also referred as to a Massive MIMO) system generally improves spectral efficiency in 5th Generation (5G) cellular communication. Through a Large-scale MIMO or a Massive MIMO, rich freedom of signal processing is utilized in the system, interference among terminals and interference among cells can be reduced, computational complexity can be reduced, and communication link quality can be improved. In addition, through the Large-scale MIMO or a Massive MIMO, power consumption of a single antenna unit can be reduced, and power efficiency of the system can be improved. Since the adopted spectrum is gradually moving from a low frequency to a high frequency (form factor of antennas is gradually decreased), a future base station device and a mobile device may adopt a much larger number of antennas than present. At present, in a prototype testing system, availability and an industrial applicability of system with more than 64 antennas has been tested. An implementation in a millimeter-wave frequency in a Large-scale MIMO system or a Massive MIMO system includes procedures as follows. When a distance between antennas is short (in a level of a half-carrier wavelength), by configuring the Large-scale MIMO system or the Massive MIMO system, a base station generates extremely-narrow transmission beams to serve multiple terminals. At the same time, a terminal can also be configured with multiple antennas to generate different gains for different beam directions and to select a reception beam with a high gain to perform data reception. When each transmission beam of the base station serves a terminal, interferences among terminals can be reduced. When base stations use different directional beam to serve their terminals, interferences among cells can be reduced. In the Large-scale MIMO system or the Massive MIMO system, when the transmitter knows a precise CDI, a signal-to-noise ratio (SNR) of the uplink channel and the downlink channel is increased in accordance with increase of the number of antennas. Thus, when there are scores of or hundreds of antennas, system capacity can be improved. When the pilot contamination occurs, an actual capacity of the Large-scale MIMO system or the Massive MIMO system can be seriously decreased. Even though a transmission power of the base station is low, the system still works in an interference-limitation state. Pilot contamination causes a serious impact for the Large-scale MIMO system or the Massive MIMO system. Thus, in order to improve system capacity, it is important to introduce a new method for obtaining CDI to solve the pilot contamination in the Large-scale MIMO system or the Massive MIMO system.
In a FDD system, since an uplink channel and a downlink channel are respectively located in different frequency bands, there is no Channel Reciprocity between the uplink channel and the downlink channel. A base station cannot obtain CDI of the downlink channel through estimating the uplink channel. In such a condition, a terminal has to occupy a part of resources of the uplink channel to feed the CDI back to the base station. A method of obtaining the CDI is through explicit feedback. The terminal quantizes the CDI of the downlink channel through a fixed codebook. A quantization result is reported to the base station through the uplink channel. Another method is implicit feedback. According to CDI of the downlink channel, the terminal selects a desired pre-code from several fixed pre-codes, and reports a selection result to the base station through the uplink channel. Regardless which method is adopted, in order to make the base station obtain precise CDI of the downlink channel, overhead occupied for uplink feedback of the terminal has to be increased in accordance with increase of the number of antennas. That is, a method that CDI is obtained based on feedback in a current FDD system cannot apply to the Large-scale MIMO system or the Massive MIMO system. That is because the number of antennas is large, and overhead of uplink feedback about the CDI of the terminal will be a load of the system.
It can be seen from the above that, in a design of the 5G communication system, it is to solve a problem of obtaining CDI in a Large-scale MIMO system or a Massive MIMO system. A fast and effective method for obtaining CDI can decrease signaling overhead and reference overhead of the system, relieve pilot contamination, ensure spectral efficiency introduced by a Large-scale MIMO system or a Massive MIMO, and improve system capacity of the cell.